The electrical power system in the United States generates three-phase alternating current (AC) electrical power. Each power phase is 120 degrees out of phase, plus or minus, with the other two power phases. The voltage of any phase oscillates sinusoidally between positive voltage and negative voltage. It happens that three-phase electrical power generation, transmission, and distribution provides an acceptable compromise between the efficiency and the expense and complexity of power system equipment.
It is more efficient to transmit electrical power at high voltage levels than at low voltage levels. Electrical power may be generated as three-phase AC power at moderate voltage levels in the 12 thousand volt (kV) to 25 kV range. The voltage level may stepped up to the 110 kV to 1000 kV range using a transformer for transmission over long transmission lines, hence minimizing transmission line power loss. The transmission line voltage may be stepped down, using a transformer at a substation, to the 12 kV to 35 kV range for local distribution. The local distribution voltage level may be further stepped down through one or more transformer stages to provide 120 volt AC power to the home and office. Special accommodations may be made for manufacturing plant electrical power consumers. In some contexts, the electrical power system may be abstractly categorized into electrical power generation, electrical power transmission over extended distances, and electrical power distribution to electrical power consumers.
Power system transformers may comprise three pairs of wire windings, one pair of windings for each phase. Each pair of wire windings is constructed so that an alternating electric current in a primary winding creates a fluctuating electro-magnetic field that couples into the secondary winding, thereby inducing a corresponding alternating electric current in the secondary winding. Typically the primary and secondary windings are wound on a common core that improves the efficiency of the transformer by concentrating the electro-magnetic field within the common core, thereby improving the coupling between the primary and secondary windings. The voltage in the secondary winding is proportional to the voltage in the primary winding, and the current in the secondary winding is inversely proportional to the current in the primary winding, where the proportionality in both cases is based on the ratio of the number of turns of wire in the secondary winding to the number of turns of wire in the primary winding. This ratio is referred to as the turns ratio. The performance of power system transformers may change as insulation of the windings deteriorates, as resistance of the winding wires changes over time or with temperature, as physical properties of the common core changes over time, and other. The ratio of number of turns in the secondary winding and the number of turns in the primary winding may change if a turn shorts at a point of insulation breakdown. A wide variety of power system transformer configurations is known, and some transformers may vary somewhat from the general description above. Some transformers may be auto-transformers. Some transformers may have taps.
Testing of power system transformers may be conducted by connecting a test set to the windings of the power system transformers and exciting the primary winding and the secondary winding with electrical signals, both direct current and alternating current. Testing may be conducted on one transformer phase at a time, or may be conducted on multiple transformer phases concurrently. From some points of view, testing generators has some similarities to testing transformers. An exciter winding in a generator may be considered to be similar, in some respects, to a transformer winding. The windings of a generator may be considered to be similar, in some respects to a transformer winding. Transporting the power system transformer or generator to a controlled test laboratory environment may not be economically feasible, and therefore testing typically occurs on site, often outdoors in variable weather conditions. As can readily be appreciated by one skilled in the power distribution art, the testing environment associated with high voltage power system transformers may be subject to intense electric field fluxes as well as high levels of air borne dust and grit.